Immune system diseases are significant health-care problems that are growing at epidemic proportions. As such, they require novel, aggressive approaches to the development of new therapeutic agents. Standard therapy for autoimmune disease has been high dose, long-term systemic corticosteroids and immunosuppressive agents. The drugs used fall into three major categories: (1) glucocorticoids, such as prednisone and prednisolone; (2) calcineurin inhibitors, such as cyclosporine and tacrolimus; and (3) antiproliferative/antimetabolic agents such as azathioprine, sirolimus, and mycophenolate mofetil. Although these drugs have met with high clinical success in treating a number of autoimmune conditions, such therapies require lifelong use and act nonspecifically to suppress the entire immune system. The patients are thus exposed to significantly higher risks of infection and cancer. The calcineurin inhibitors and steroids are also nephrotoxic and diabetogenic, which has limited their clinical utility (Haynes and Fauci in Harrison's Principles of Internal Medicine, 16th edition, Kasper et al., eds (2005), pp 1907-2066).
In addition to the conventional therapies for autoimmune disease, monoclonal antibodies and soluble receptors that target cytokines and their receptors have shown efficacy in a variety of autoimmune and inflammation diseases such as rheumatoid arthritis, organ transplantation, and Crohn's disease. Some of the agents include infliximab (REMICADE®) and etanercept (ENBREL®) that target tumor necrosis factor (TNF), muromonab-CD3 (ORTHOCLONE OKT3) that targets the T cell antigen CD3, and daclizumab (ZENAPAX®) that binds to CD25 on activated T cells, inhibiting signaling through this pathway. While efficacious in treating certain inflammatory conditions, use of these drugs has been limited by side effects including the “cytokine release syndrome” and an increased risk of infection (Krensky et al., in Goodman and Gilman's The Pharmacological Basis of Therapeutics, 10th edition, Hardman and Limbird, eds, (2001), pp 1463-1484).
Passive immunization with intravenous immunoglobulin (IVIG) was licensed in the United States in 1981 for replacement therapy in patients with primary antibody deficiencies. Subsequent investigation showed that IVIG was also effective in ameliorating autoimmune symptoms in Kawasaki's disease and immune thrombocytopenia purpura (Lemieux et al., Mol. Immunol., 42:839-848, 2005; Ibanez and Montoro-Ronsano Curr. Pharm. Biotech., 4:239-247, 2003; Clynes, J. Clin. Invest., 115:25-27, 2005). IVIG has also been shown to reduce inflammation in adult dermatomyositis, Guillian-Barre syndrome, chronic inflammatory demyelinating polyneuropathies, multiple sclerosis, vasculitis, uveitis, myasthenia gravis, and in the Lambert-Eaton syndrome (Lemieux et al., supra; Ibanez and Montoro-Ronsano, supra).
IVIG is obtained from the plasma of large numbers (10,000-20,000) of healthy donors by cold ethanol fractionation. Commonly used IVIG preparations include Sandoglobulin, Flebogamma, Gammagard, Octagam, and Vigam S. In general, efficacy is seen when only large amounts of IVIG are infused into a patient, with an average dose of 2 g/kg/month used in autoimmune disease. The common (1-10% of patients) side effects of IVIG treatment include flushing, fever, myalgia, back pain, headache, nausea, vomiting, arthralgia, and dizziness. Uncommon (0.1-1% of patients) side effects include anaphylaxis, aseptic meningitis, acute renal failure, haemolytic anemia, and eczema. Although IVIG is generally considered safe, the pooled human plasma source is considered to be a risk factor for transfer of infectious agents. Thus, the use of IVIG is limited by its availability, high cost ($100/gm, including infusion cost), and the potential for severe adverse reactions (Lemieux et al., supra; Ibanez and Montoro-Ronsano, supra; Clynes, J. Clin. Invest., 115:25-27, 2005).
Numerous mechanisms have been proposed to explain the mode of action of IVIG, including regulation of Fc gamma receptor expression, increased clearance of pathogenic antibodies due to saturation of the neonatal Fc receptor FcRn, attenuation of complement-mediated damage, and modulation of T and B cells or the reticuloendothelial system (Clynes, supra). Since Fc domains purified from IVIG are as active as intact IgG in a number of in vitro and in vivo models of inflammation, it is well accepted that the anti-inflammatory properties of IVIG reside in the Fc domain of the IgG (Debre et al., Lancet, 342:945-949, 1993) or a sialylated subfraction (Kaneko et al., Science, 313:670-673, 2006).
Fc receptors for IgG (FcγR) play a unique role in mammalian biology by acting as a bridge between the innate and the acquired immune systems (Dijstelbloem et al., Trends Immunol. 22:510-516, 2001; Takai, Nature 2: 580-592, 2002; Nimmerjahn and Ravetch, Immunity 24: 19-28, 2006). By virtue of their binding to the Fc region of IgG (Woof and Burton, Nature Rev. Immunol., 4:1-11, 2004), FcγR regulate a variety of effector functions in ADCC, complement-mediated cell lysis, type III hypersensitivity reactions, tolerance, phagocytosis, antigen presentation, and the processing and clearance of immune complexes (Dijstelbloem et al., supra; Takai, supra; Nimmerjahn and Ravetch, supra).
The FcγR comprise three major gene families in humans including FcγRI (CD64), FcγRII (CD32), and FcγRIII (CD16) (Dijstelbloem et al., supra; Takai, supra). FcγRI is a high affinity receptor for monomeric IgG (108-109 M−1) where FcγRII and FcγRIII exhibit low affinities for monomeric IgG (107 M−1) but bind to IgG immune complexes with greatly increased avidities. The FcγRII subfamily is composed of two major classes of genes, FcγRIIa and FcγRIIb, which after binding IgG transmit opposing signals to the cell interior. FcγRIIa contains an immunoreceptor tyrosine-activating motif (ITAM) within its short cytoplasmic tail, while FcγRIIIb transmits inhibitory signals through an immunoreceptor tyrosine inhibitory motif (ITIM) within its cytoplasmic domain. FcγRIII subfamily also contains two distinct receptor genes, FcγRIIIa and FcγRIIIb. FcγRIIIa is a heterodimeric signaling receptor that after binding IgG immune complexes transmits activating signals through its associated ITAM-containing common γ chain. FcγRIIIb is bound to the cell membrane through a GPI linker and lacks intrinsic signaling capacity. FcγRI also lacks an intrinsic signaling capacity but similar to FcγRIIIa, associates with the common γ chain to transmit activating signals upon Fc binding. Signaling through FcγR involves kinase mediated phosphorylation/dephosphorylation events within the ITAM/ITIM sequences (Dacron, Intern. Rev. Immunol., 16: 1-27, 1997).
Consistent with their reported roles in immune biology, the human FcγR exhibit different affinities for subclasses of monomeric IgG: FcγRI binds IgG1≧IgG3>IgG4>>IgG2; FcγRIIa binds IgG3≧IgG1, IgG2>>IgG4; FcγRIIb binds IgG3≧IgG1>IgG4>IgG2; FcγRIIIa and FcγRIIIb bind IgG1, IgG3>>IgG2, IgG4 (Dijstelbloem et al., supra; Takai, supra).
In addition to differences in structure and signaling capacities, the FcγR also exhibit differences in cellular expression patterns. In humans, FcγRI is expressed predominantly on macrophages, monocytes, and neutrophils but can also be found on eosinophils and dendritic cells. FcγRIIa is the most widely expressed FcγR in humans and is expressed on platelets, macrophages, neutrophils, eosinophils, dendritic cells and Langerhans cells. FcγRIIb is the only FcγR expressed on B cells but is also expressed by mast cells, basophils, macrophages, eosinophils, neutrophils, dendritic and langerhan cells. FcγRIIIa is the only FcγR expressed on human NK cells and is widely expressed, found on macrophages, monocytes, mast cells, eosinophils, dendritic and langerhan cells. The expression of FcγRIIIb, on the other hand is largely restricted to neutrophils and eosinophils (Dijstelbloem et al., supra; Takai, supra).
Mice express FcγR that function similarly to the receptors in humans such as the orthologs of human high affinity FcγRI and the inhibitory receptor FcγRIIb (Nimmerjahn and Ravetch, Immunity, 24:19-28, 2006). The murine orthologs of human FcγRIIa and IIIa are thought to be FcγRII and FcγRIV, respectively. Mice do not appear to express FcγRIIIb (Nimmerjahn and Ravetch, supra). Although some differences in cellular expression patterns have been noted, FcγR gene expression in humans and their orthologs in mice are generally similar.
Gene targeting in mice has suggested the importance of FcγR in the mammalian immune system (see generally Dijstelbloem et al., supra; Takai, supra; Nimmerjahn and Ravetch, supra). Deletion of the common γ chain, the signaling subunit of FcγRI, FcγRIII, and FcγRIV, abolishes signaling through all activating FcγR and renders mice resistant to a variety of autoimmune and inflammatory conditions. Mice deficient in the γ-chain exhibit attenuated immune complex-alveolitis, vasculitis, glomerulonephritis, Arthus reaction, and autoimmune hemolytic anemia. Similar data have been described for deletion of the α-chains of FcγRII and FcγRI. FcγRIII−/− mice exhibit reduced immune complex-induced alveolitis, reduced sensitivity to autoimmune hemolytic anemia and an attenuated Arthus reaction. FcγRI−/− mice show impaired phagocytic function of macrophages, decreased cytokine release, attenuated ADCC and antigen presentation, reduced arthritis, enhanced antibody responses, and impaired hypersensitivity. Deletion of the inhibitory receptor, FcγRIIb, in contrast, results in augmented inflammation and autoimmune responses. FcγRIIb−/− mice show enhanced collagen-induced arthritis, spontaneous development of glomerulonephritis on a C57BL/6 background, enhanced Arthus reaction, enhanced alveolitis, enhanced IgG-induced systemic anaphylaxis, and enhanced anti-GBM induced glomerulonephritis. Thus, the FcγR play key roles in immune system homeostasis.
There is a need for Fc receptor antagonists, including FcγRI antagonsists, useful in treating a variety of autoimmune diseases. Specifically, such antagonists would function to regulate the immune and hematopoietic systems, since disturbances of such regulation may be involved in disorders relating to inflammation, hemostasis, arthritis, immunodeficiency, and other immune and hematopoietic system anomalies. Therefore, there is a need for identification and characterization of such antagonists that can be used to prevent, ameliorate, or correct such disorders.